Ion Gun basics
Posted: Fri Aug 24, 2012 11:22 pm
I have started a new thread so as not to muddy Richards excellent FAQ
This is primarily aimed at Preston
Ion guns are used so as to provide a charged particle that is looking for something of the opposite charge hence they tend to move towards this opposite charge. The rate at which they move depends on what potential they differ from the other electrode.
With ions having the same charge they naturally want to repel from each other and therefore the ion beam expands (defocuses).
The slower the ion beam the more time they have to repel from each other so lower potential beams defocus further than beams at high potential.
Another thing that defocuses beams is the ions hitting neutral gas atoms (mean free path )--so the better the vacuum the better the eventual spot.
Now on to Prestons inquiry
a 100mA beam is considered a big beam 10kV is considered a low accelerating potential so the chance of getting this number of ions into a 2mm spot is not possible. Also the vacuum of only a micron is such that the mean free path is only a few mm so there is yet another defocusing mechanism. Also a 100mA @ 10kV over a 2mm spot is a heat load of 33 kWatt per sq cm.
Check out what 16 watts per sqcm looks like with a target stage cooled to -179 degrees celsius
Note that in the following the vacuum in the beam line is 1x 10 to the minus 7 torr
http://www.youtube.com/watch?v=oALDWvav ... ults_video
This is primarily aimed at Preston
Ion guns are used so as to provide a charged particle that is looking for something of the opposite charge hence they tend to move towards this opposite charge. The rate at which they move depends on what potential they differ from the other electrode.
With ions having the same charge they naturally want to repel from each other and therefore the ion beam expands (defocuses).
The slower the ion beam the more time they have to repel from each other so lower potential beams defocus further than beams at high potential.
Another thing that defocuses beams is the ions hitting neutral gas atoms (mean free path )--so the better the vacuum the better the eventual spot.
Now on to Prestons inquiry
a 100mA beam is considered a big beam 10kV is considered a low accelerating potential so the chance of getting this number of ions into a 2mm spot is not possible. Also the vacuum of only a micron is such that the mean free path is only a few mm so there is yet another defocusing mechanism. Also a 100mA @ 10kV over a 2mm spot is a heat load of 33 kWatt per sq cm.
Check out what 16 watts per sqcm looks like with a target stage cooled to -179 degrees celsius
Note that in the following the vacuum in the beam line is 1x 10 to the minus 7 torr
http://www.youtube.com/watch?v=oALDWvav ... ults_video